In the semiconductor manufacturing art, integrated circuits (IC's) may be constructed and tested and the resulting yield of semiconductor circuits fall within a distribution curve of performance characteristics. For example, a number of IC's may operate up to a particular clock speed without significant error, while other IC's, from the same manufacturing batch, may operate only at lower clock speeds, without significant error. Small variations in the size and shape of various semiconductor structures in the IC may alter how the IC performs at various clock speeds and temperatures. In the prior art, if an IC tested cannot perform at a given clock speed, it may have been marketed as capable of a lower clock speed, and thus reduce scrappage rates.
However, IC's may need to operate both within an acceptable limit of temperature as well as clock speed. As temperature increases, an IC may operate in a different manner, cease to operate reliably, or cease to operate at all. An IC manufacturer may need to certify that its products are capable of operating within a predetermined temperature range, and each IC may need to be tested to insure that the device actually meets the temperature range specification. As with clock speed, operating temperature range for an individual IC within a batch may vary, due to manufacturing tolerances and the like. Thus, it may be necessary to test each IC for compliance with temperature design characteristics.
Another concern with IC design and testing is that many IC's require accurate voltage references, in order to operate with precision. Analog-to-digital converters, for example, require a precise voltage reference Vref, to compare with an input signal. This voltage reference Vref should not drift with temperature, or if it does, such drift needs to be corrected or compensated. For precision analog-to-digital converters a voltage reference known as a bandgap voltage reference may be employed. A bandgap voltage reference is a largely temperature independent voltage reference circuit widely used in integrated circuits, usually with an output voltage around 1.25 V. This circuit concept was published by David Hilbiber in 1964. (See, e.g., Hilbiber, D. F. (1964), “A new semiconductor voltage standard”, 1964 International Solid-State Circuits Conference: Digest of Technical Papers 2: 32-33, incorporated herein by reference).
FIG. 1 is a schematic of a typical bandgap voltage reference, provided for purposes of illustration. The voltage difference between two p-n junctions (e.g. transistors T1, T2), operated at different current densities (10E, E) is determined in adder S, and the output is the voltage reference Vref. The resulting voltage is about 1.2-1.3 V, depending on the particular technology and circuit design. The remaining voltage change over the operating temperature of typical integrated circuits is on the order of a few millivolts. This temperature dependency has a typical parabolic behavior, as will be illustrated below.
While a bandgap voltage reference may be very accurate and largely insensitive to temperature changes, for high-precision applications or applications where small voltages are measured, even this variation of a few millivolts may not be acceptable. In order to compensate for this slight variation with temperature, a temperature compensation correction may be added to the output of the bandgap voltage reference, (or may be corrected further downstream in the circuit) to correct for temperature drift. Typically, a correction curve may be plotted, using a minimum of three reference points needed to define the correction curve.
However, applying a standard correction curve to a bandgap voltage reference may not provide accurate temperature compensation, as each IC in a batch may have slightly different characteristics. Thus, it may be necessary to test each IC at various temperature points and obtain data to generate a temperature correction curve for the individual IC. This temperature curve may then be programmed into the device, for example in a one-time programmable (OTP) IC.
In the prior art, the technique for temperature-testing such IC's was slow and cumbersome. An individual IC would be placed in an oven or other type of heating device and temperature slowly raised and the functionality of the IC verified, or in the case of a bandgap voltage reference, temperature compensation data recorded. For initial design and prototyping, such slow and cumbersome techniques were acceptable in determining the overall design characteristics of the IC. However, for production testing—where large quantities of IC's need to be tested for temperature tolerance en masse, such techniques are not as workable, as the time required for testing is too long. In production testing, a test cycle on the order of seconds may be desirable.
For production calibration purposes, the techniques of the prior art have significant room for improvement. Heating an IC to a desired temperature point and waiting for the temperature to stabilize at that point (i.e., “stair stepping”) might take on the order of 1.5 to 2 seconds or more, per data point. Thus, each point in the calibration curve may take a second or two to obtain. If three points are required (at minimum) to define the calibration curve, then it may take six seconds or more to obtain the calibration curve for a production part. In production high-speed testing circumstances, such a delay is usually not acceptable, as other tests also need to be performed on the IC.
If an external heat source is used to ramp up the temperature of the IC at the tester then the heat source will also heat up the test socket and other IC's (or subsequent IC's) on the test board. There would need to be an additional time allocated to bring the board and test setup temperature to nominal value before testing the next IC. This adds a significant amount of time to the total test time, which is not acceptable for production testing. Moreover, most production testers do not have such heating capabilities, likely because of these concerns.
A need exists in the art to provide an improved technique for testing IC's at various temperature points, both quickly and inexpensively, in order to allow for each IC in production to be tested and/or calibrated.